Exemplary embodiments of the present invention relate to fiber-reinforced composite materials useful for applications where a high strength-to-density ratio is demanded. For example, this applies for structural components of aerial vehicles as e.g. airplanes and helicopters.
A conventional fiber-reinforced composite material comprises a matrix and reinforcing fibers embedded therein. The matrix is often a thermally curable resin, possibly with particular additives (“resin system”). The reinforcing fibers are usually provided in form of dry or pre-impregnated fiber mats, which are processed in an infusion process or a prepreg process, respectively. Such processing can take place in a forming tool (mold), in which a plurality of fiber mats, as e.g. woven fabrics or non-crimp fabrics, are stacked and cured for manufacturing the fiber-reinforced composite material.
Depending on the material used to create the matrix, many fiber-reinforced composite materials have insufficient a fire performance for their applications. Conventional materials as e.g. carbon fiber reinforced plastics (CFRP) may ignite and burn, releasing heat, smoke and toxic gases. In addition, with increasing temperature caused by fire the structural integrity of the structure is decreased. Consequently, loaded composite structures often collapse within a short period of time, when subjected to fire. This is particularly notable when the stresses present are compressive because the compressive strength falls to a very low value when the matrix reaches its softening point.
Exemplary embodiments of the present invention are directed to a fiber-reinforced composite material with improved fire resistance.
The inventive fiber-reinforced composite material comprises a “bulk portion” comprising a bulk matrix with reinforcing fibers embedded therein, and a “surface portion” comprising a plurality of laminated first layers, which expand and/or delaminate during fire impact, preferably above 100% and more preferably above 1000%. In an embodiment, the surface portion is clearly distinguishable from the bulk portion, i.e. has a structure that is different from the structure of the bulk.
In this material, the bulk portion can have a structure of any kind known from the prior art. In a preferred embodiment, the bulk portion is a multilayer structure comprising a plurality of fiber material layers (e.g. tissues, non-crimp fabric, unidirectional fibers etc.) embedded in a resin matrix (bulk matrix). This matrix is preferably an epoxy system or any other resin, e.g. thermally curable resin. Further, the matrix may be a thermoplastics. In other embodiments, the bulk matrix is a system based on benzoxazine or cyanate ester. The latter matrix materials are in particular of interest in aeronautic applications. The reinforcing fibers embedded in the bulk matrix, preferably arranged in the form of fiber mats, may be carbon fibers, glass fibers or other fibers.
It is essential for the invention that such a bulk portion (“bulk composite”) is supplemented by additional fire protecting layers, e.g. in form of a separate surface portion (“surface laminate”) comprising a plurality of laminated layers, which expand and/or delaminate during fire impact. These layers are referred to as “first layers” or “expanding layers” in the following.
Such a surface portion or “surface laminate” can be provided on one or both surfaces of a sheet-like bulk composite, creating a fire protection for the underlying bulk composite. In another embodiment, the “surface laminate” has the same structure as the “bulk portion”, i.e. has reinforcing fibers embedded therein and comprises a plurality of laminated first layers, which expand and/or delaminate during fire impact. Thus, in particular, the “surface laminate” may be identical to the “bulk portion”.
When exposed to the heat of fire, the expanding layers will expand and/or delaminate. In this way, a remarkable decrease in the thermal conductivity can be achieved, so that the progress of heat to the underlying bulk portion is slowing down. Thus, the inventive material withstands fire to a greater extent, and maintains structural integrity (in the bulk portion) for an extended period of time.
Preferably, the surface laminate has in case of foils (used in the surface laminate) a thickness of at least 1%, in particular at least 2%, and in case of fiber reinforcements up to 20% of the thickness of an underlying bulk portion. On the other hand, the thickness of the surface laminate is preferably less than 100%, in particular less than 10% of the thickness of the underlying bulk portion. However, as mentioned above, the “first layers” can also be integrated into the “bulk portion”, so that the surface portion may be identical to the bulk portion.
In an embodiment, at least one of the expanding layers comprises a thermoplastic film. Preferably, the thermoplastic is a polymer selected from the group consisting of Polyimide (PI), Polyetherimide (PEI), Polyether sulfone (PES), Polyamide (PA), Polyamide-imide (PAI), Polysulfone (PSU), Polyphenylsulfone (PPSU), Polyetherketone (e.g. PEEK, PEKK), Polyethylene terephthalate (PET), Polyethylene (PE), Polyester, Polyetherester (PEE), Polyesteramide, Polymethylmethacrylate (PMMA), Polypropylene (PP), Polystyrene (PS), Polyvinylchloride (PVC) or mixtures thereof.
Preferably, the thermoplastic is selected or modified so that it is able to expand at a temperature, e.g. above its glass transition or melting temperature, which is higher (e.g. in the range of 200 to 240° C.) than the processing temperatures in the manufacturing process, and higher than the service temperatures in the later application.
Preferably, the thermoplastic begins expansion at a temperature of less than 400° C., preferably less than 250° C. In this case, the expansion takes place in the early stages of fire.
The thermoplastic (or polymeric) film may be perforated (provided with apertures as holes and/or slits) to allow the matrix material to be distributed throughout the surface laminate in the production process thereof. For example, a needle or laser perforation can be used to create suitable apertures in the film, improving the infiltrating flow of the matrix material (e.g. epoxy system). To this end, e.g. a hexagonal or square perforation pattern can be provided (perforation distance e.g. 1 to 5 cm, perforation diameter e.g. 0.5 to 2 mm). Slitting or perforation of the film can also be used to increase the draping over curvatures, i.e. increase the drapability.
In an embodiment, at least one of the expanding layers comprises fibers (preferably carbon or glass fiber), e.g. short fibers dispersed in a plastic film, or in the form of a non-woven fabric (e.g. a fleece) embedded in a plastic. Also continuous fibers (unidirectional or woven) can be used.
In an embodiment, at least one of the expanding layers comprises an intumescent modifier, e.g. a foaming agent, expandable graphite and/or a phosphorous product (e.g. Ammoniumpolyphosphate). Such modifiers can advantageously promote the expansion of the respective expanding layer.
In an embodiment at least one of the expanding layers has a thickness in the range of 5 to 300 μm, preferably 10 to 200 μm, more preferably 20 to 130 μm, in particular 125 μm. The composite of an embodiment having expanding layers with a thickness of 125 μm or 125±25 μm has a lower heat release rate and ignites at a later time than composites having thinner or thicker expanding layers.
The surface laminate can comprise e.g. one, two, three, four, or even more expanding layers.
In a preferred embodiment, the surface laminate consists not only of expanding layers (first layers), but further comprises a plurality of laminated layers (second layers) different from the first layers.
The second layers can be designed to have the function of decreasing a transfer of heat, to prevent oxygen (air) to diffuse to the bulk matrix and/or to prevent combustible products from diffusing from the bulk matrix to the outside (ignition zone). Therefore, the second layers are also referred to as “barrier layers” in the following.
With expanding layers and barrier layers as mentioned above, the surface laminate can have a multi-material design comprising of these two different types of layers.
Preferably, the expanding layers and the barrier layers are stacked in an alternating sequence to form the surface laminate. In particular, such sequence may comprise at least one expanding layer and one barrier layer, or e.g. at least two expanding layers and two barrier layers. One example would be the sequence barrier-expanding-barrier-expanding, wherein the outer side of the surface laminate is preferably formed by a barrier layer and the inner side (interface to the bulk composite) is preferably formed by an expanding layer. Other arrangements like e.g. a combination of different barrier layers for synergistic effects is also possible. Another possible arrangement is the combination of expanding layers with different expansion behavior and/or different expansion temperature to optimize the protection of an underlying bulk material. Another possible arrangement are fiber reinforced thermoplastic or fiber reinforced resin layers between or on top of the barrier layers or/and expansion layers, possibly modified, to act as additional barrier, expansion, stabilizing or structural layer.
The alternating layers, described above may also be used throughout the complete material thickness.
In an embodiment, at least one of the barrier layers comprises a matrix (“surface matrix”) with reinforcing fibers embedded therein. In this way, the respective barrier layers can advantageously contribute to the mechanical strength of the material. There may be used short fibers (e.g. dispersed in the surface matrix) or continuous fibers in form of a fiber mat. The surface matrix may be the same material as that of the bulk matrix (e.g. an epoxy resin), or another material. In an embodiment, the barrier layers comprise thermoplastic films with continuous fibers (e.g. carbon fibers), unidirectional or woven, e.g. embedded in a PEI prepreg. As a resin, e.g. cyanate esters or resins with high inorganic content (e.g. Si) might be used.
In an embodiment at least one of the barrier layers comprises a polymer. For example, thermoplastic polymers as mentioned above for the expanding layers, in particular Polyimide (PI), Polyamide-imide (PAI), Polyetherketone (e.g. PEEK, PEKK) or Polyphenylsulfone (PPSU) may be used as material for creating barrier layers. Upon exposure to heat, the latter materials form remarkable amounts of soot (decomposition products), which may advantageously contribute to the above mentioned barrier effect.
When a thermoplast polymer or thermosetting polymer is used for a barrier layer, this polymer may also be reinforced by a fiber material (short or continuous fibers).
Alternatively or in addition to such fiber material, at least one of the barrier layers can comprise inorganic or organic particles dispersed in a matrix. This matrix can also be a resin matrix. Preferably, microparticles with an average particle size of less than 100 μm, or nanoparticles with at least one dimension smaller than 100 nm are used. In an embodiment, the barrier layer comprises an inorganic filled epoxy matrix. Preferably, inorganic particles as e.g. phyllosilicates (layered silicates) are used. But each barrier layer may also be modified with other additives or fillers. Preferably, organic particles as e.g. organophosphorous, halogenated, paraffinic, spumific and/or intumescent compounds, polymers or cork are used. PCT patent document WO 2010/020237 describes particles (in a thermoplastics) that can also be used in the present invention as particles in the barrier layer.
In another embodiment, at least one of the barrier layers is a metallic layer.
Each barrier layer may be perforated. To this end, e.g. a hexagonal or square perforation pattern can be provided (perforation distance e.g. 1 to 5 cm, perforation diameter e.g. 0.5 to 2 mm).
As to the process for manufacturing the fiber-reinforced composite material, methods and tools can be applied, which are well known from the fiber composite technology. Examples are transfer molding (RTM, “resin transfer molding”), vacuum-infusion (such as VAP, VARI etc.) and their further developments (such as SLI, LRI, BP-RTM), as well as “Prepregging”.
For manufacturing the inventive material, the layers creating the later surface composite can be stacked together with a dry preform or prepreg (one or more fiber material layers) creating the later bulk composite in a forming tool. Then, in case of a dry preform, an infusion process can be provided to infuse the dry preform, and as the case may be also the layers of the surface portion, with liquid or semi-liquid matrix material (e.g. resin as e.g. epoxy resin system). Finally, a curing process (e.g. thermal curing) can be used to bond all layers together (co-curing or co-bonding, respectively).
A thermoplastic film might also partially dissolve to increase the interaction with the resin.
In an alternative processing, at least some (or all) layers creating the bulk portion and the surface portion can be “prepregged” (impregnated) with the respective matrix material before they are stacked in the forming tool (prepreg process). In this case, the expanding, barrier and bulk layers can be laid in the desired order in the tool, and cured (under pressure) afterwards.
Preferably, expanding layers and barrier layers are stacked in an alternating sequence on one or both sides of a multi-layer bulk portion. A typical layout is for example: barrier-expanding-barrier-expanding-bulk. Instead of two expanding layers and two barrier layers in this example, there can be used also more (e.g. three or four) expanding layers and/or barrier layers, up to 100% of the laminate (composite material).
In an embodiment, a barrier layer and expanding layer are combined in advance before the stacking process is carried out. Such a combined expanding-barrier-layer may provide a “soot forming expanding layer”. Typical technique for the formation of such a soot forming expanding layer is calendaring (in case of two thermoplastic layers), or “doctorblading” in case of a thermoset and thermoplast combination.
With a curing process the fiber-reinforced composite material is completed.
The inventive material can be advantageously used to form a structural component for an aerial vehicle, in particular for an airplane or helicopter. In an embodiment, the structural component is a fuselage shell or hull section of the respective vehicle.
An important feature of the invention is that when not in the intumesced state, the added material (surface portion) can be designed to be able to share the load carried by the e.g. underlying (bulk) structure. The inventive protective layers does not, therefore, add to the weight of the structure without increasing the strength.
A further important feature of the invention is that when at least one of the expanding layers comprises a thermoplastic film arranged in alternating manner with the barrier layers, although the ignition of the composite start early compared to the state of the art, the heat release is significantly lower, e.g. about 30% to 50% depending on the thickness of the expanding and/or barrier layers. In other words, even if the ignition starts earlier, the heat release and the decomposition for a composite as described by the invention, is significantly lower as for state of the art composites; in addition the time until the bulk material of the composite is decomposed is about 1.5 to 2 times longer than within the state of the art. Thus, the heat from the burning process is released later, so that the flame spread is slower, the heat is lower and the remaining mechanical properties are maintained, as the material is less damaged and fails later.